The invention relates to methods and apparatus for transmitting multiple signals over large numbers of channels, where two or more of the channels are affected by or transmitted by a component which produces intermodular distortion from the interaction of the signals of these channels. One kind of such component is a traveling wave tube amplifier which may be used as a transmitter output amplifier carrying many channels of information. However, the invention may also be applicable to any other transmission system transmitting more than one channel with one transmitter, because known active electronic devices all introduce at least some distortion.
A system of this type, which has been operated publicly for many years for transmitting multichannel television as described in U.S. Pat. No. 4,747,160, transmits up to 50 television channels in the 28 GHz band. Two traveling wave tube amplifiers (TWTA's) are used for the transmitter output, one amplifying the channels in the lower half of the band, and the other amplifying the channels in the upper half of the band. Because intermodular distortion in a TWTA is a function of its power level, the TWTA's in this system are normally driven to an output power well below their full rated output. While this system has been operating successfully on a commercial basis, with signal quality that exceeds that of typical cable TV, improvements which will permit greater output power from the TWTA with acceptably low signal distortion or noise in the customer's TV picture are always desirable.
Because it simplifies providing equipment which is user friendly, and such matters of administration as setting standards, historically multi-channel systems in a particular band always space channels for a given class of service equally, usually producing carrier frequencies which have integral, easily remembered numbers. It is typical that this is done despite the need for efficient utilization of available spectrum.